1. Field of the Invention
The present invention relates to a system and method for measuring the leakage impedance and loss angle of a high voltage component or insulation system to determine the condition of the component or system as part of the installation and maintenance procedure.
2. Description of the Related Art
As known to those skilled in the art, the evaluation of electrical insulation in power systems and power system components is of great importance when performing any number of tasks, such as scheduling preventive maintenance, qualifying a power system apparatus going into service, or trouble-shooting an apparatus to determine the cause of an unknown failure or problem.
In such cases, the condition of insulation systems and components can be assessed by measuring the loss angle, or δ, which gives a measure of the performance of the insulation. An ideal insulation system behaves as an ideal capacitor in that, when the system is energized with an alternating voltage, the current that flows in the insulation system is exactly 90° out of phase with the voltage.
A real insulator however, has a finite resistance that appears in parallel with this ideal capacitance value, which causes an energy loss when the system is energized as more clearly shown in FIG. 1. In FIG. 1, a real insulator is illustrated as a model schematic 100 and vector diagram 110, having an ideal capacitance 102 and a parallel finite resistance 104. This resistance 104 reduces the phase angle of the current with respect to the voltage, and the angle of this phase shift is the loss angle (δ) as shown in the vector diagram 110. As shown in diagram 110, as R approaches zero (i.e., IR increases), the greater the loss angle (δ) becomes. The loss is normally measured in terms of tan (δ), which is defined by the following equation (1),
                              Dissipation          ⁢                                          ⁢          Factor          ⁢                                          ⁢                      (                          Loss              ⁢                                                          ⁢              Factor                        )                          ⁢                                  ⁢                                  ⁢                  DF          =                                    tan              ⁢                                                          ⁢              δ                        =                                                            P                  R                                                  Q                  C                                            =                                                                    I                    R                                                        I                    C                                                  =                                                                            X                      C                                        R                                    =                                      1                                          ω                      ⁢                                                                                          ⁢                      CR                                                                                                                              (        1        )            wherein C is the capacitance of the insulation system model (i.e. 102), and R is the loss resistance of the insulation system model (i.e. 104). The term “power factor” is also often used and corresponds to equation (1) as defined by the following equation (2),
                              Power          ⁢                                          ⁢          Factor                ⁢                                  ⁢                                  ⁢                  PF          =                                    cos              ⁢                                                          ⁢              φ                        =                                          IR                I                            =                                                PR                  SC                                =                                                      tan                    ⁢                                                                                  ⁢                    δ                                                                              1                      +                                                                        tan                          2                                                ⁢                        δ                                                                                                                                                    (        2        )            One of the indicators of a deteriorating insulation system or component is that the value of tan (δ) is increasing over time.
The value of tan (δ) has been measured in the past typically using bridge balancing methods such as the Schering bridge, or an inductively coupled ratio arm bridge (e.g., Tettex type 2805). Still other test systems have used direct measurement of a voltage and current value, and then provide electronic processing of the resulting signals to measure the tan (δ) factor. One of the limitations of these methods, however, is the presence of power line frequency fields that induce spurious voltages and currents, also known as interference signals, onto the test object and/or the test system. These prevent a true measurement of the loss angle (δ) from being made.
A number of techniques for reducing the effect of this interference have been developed. The simplest of these techniques is to choose a frequency that is slightly away from the line frequency and using this to energize the test object. The voltage and current are then measured using synchronous detection or interference of the waveforms. This method relies on the assumption that the loss angle (δ) is constant with respect to frequency, which cannot be guaranteed.
An alternative technique is to measure values on either side of the line frequency and thereafter, perform a linear interpolation of the results to establish the loss angle (δ) at the line frequency. This technique has typically required the use of complex synchronous schemes requiring accurate phase shift elements, or multiple discrete measurements with complex digital signal processing noise suppression.
Each of these methods however, does not allow measuring values at line frequency. Accordingly, a need exists for a system and method to measure values, such as the leakage impedance and loss angle (δ) of a high voltage component or insulation system at a line frequency, while minimizing the effects of interference signals.